Dual-band mobile stations entered the market when single-band mobile station systems could no longer respond to the growth of mobile telephony traffic. The dual-band system consists of two networks, operated at different frequency ranges and capable of handling traffic between mobile stations and base transceiver stations. A dual-band mobile station can receive and transmit messages on both frequency bands. The choice of frequency band depends on the traffic loading and reception capabilities of each band.
A typical dual-band system will use the 900-MHz GSM network frequency for one band and the 1800-MHz frequency for the other. The 1800-MHz network is usually called DCS (Digital Cellular System); it is also called GSM-1800, but in this context the term DCS will be used. The use of two frequency bands will make possible an increase in mobile station customers when compared to a single-band system. As the GSM network is on its own no longer capable of meeting the requirements of all customers, the operators have started favouring the 1,800-MHz band by transferring mobile station calls there from the GSM frequency band.
In practice, operators try to transfer calls from GSM to DCS whenever possible, even when the GSM signal level is sufficient for transmitting the call. The measured information used for allocating the channel is received through the BCCH (Broadcast Control Channel). There are systems with a BCCH on both the DCS and GSM frequencies, but systems also exist with a BCCH on one frequency range only. The subject of the invention is specifically such systems with one BCCH, particularly dual-band networks that do not have the measured information of the BCCH available for channel allocation on the DCS side.
Single BCCH systems use fixed threshold values based on average measurements. However, the use of average values does not provide correct information on the actual signal levels of the DCS band channels. Theoretically, the difference between the GSM and DCS signal levels is 6 dB. However, the operator can select the default signal level difference for the network, because the practical difference may vary depending on the network structure and environmental conditions.
Let us study an example of frequency band limits as illustrated in FIG. 1. If the (lower) limit of signal level on the GSM band for changing the frequency band is −80 dBm, the DCS signal level must, when using the theoretical, fixed threshold value, be −86 dBm or better for the call to be continued. The GSM band signal levels on which a transfer to DCS will be effected have been marked with diagonal lines on the figure. It can be seen that if the GSM signal level is below the −80 dBm limit, no attempt will be made to transfer the connection to the DCS side, as changing the frequency band in this case would, most likely, merely lower the call quality. The shaded area of the figure is the area where the DCS network will accept the signal level.
Let us assume a situation where we want to transfer the call from GSM to DCS (1A). If, however, the signal level on the DCS band is not sufficient for maintaining the connection, the network will attempt to transfer it back to the GSM band (1B). If the signal level is still above the threshold value for the GSM band, a further attempt will be made to move the call to the DCS band (2A), and so on . . . . We have a situation where the frequency band is being unnecessarily changed back and forth, resulting in deteriorating call quality and even in losing the connection. To prevent such problems, a well-known technique is to use a timer that delays the next possible change of frequency band. However, the problem with the timer is the difficulty in setting a suitable delay. If the delay is too long, frequency changes will take too long to execute, increasing the loading on network resources. If, on the other hand, the delay is too short, there is a risk of too frequent frequency band changes. The objective of the invention is to eliminate these drawbacks.